This invention relates to a current mirror circuit and, more particularly, to an improved current mirror circuit wherein current supplied to one output terminal is substantially identical to the current which is supplied to another output terminal; and to an application for such an improved current mirror circuit.
The so-called current mirror circuit is a device generally formed of at least two stages, or circuits which, ideally, have identical currents flowing therethrough. In the current mirror circuit, changes in one current are reflected by identical changes in the other current. Thus, if one current is determined by other devices, such as an amplifier, the level of the other current is a generally accurate measure of the first current.
The current mirror circuit is a useful device for providing a current which is to be measured or used in additional apparatus but where the source of that current would be deleteriously affected or otherwise influenced if the measuring circuits or other apparatus were to be coupled to the current source. One example of such a use is in a demodulator for a composite stereo signal comprised of a main channel (L+R) signal and a sub-channel (L-R) signal modulated onto a subcarrier, which stereo signal is to be demodulated into left (L) and right (R) channel signals. One type of stereo demodulator includes an amplifier for recovering the main channel (L+R) signal and a switching circuit for recovering the oppositely phased versions of the sub-channel signal, with the main channel signal being added to both phases of the sub-channel signal. If the recovered main channel signal is added directly to the recovered sub-channel signal, it is possible that the load on the main channel amplifier would affect the recovered main channel signal level. Hence, the quality of the derived left (L) and right (R) audio channel signals would be degraded. However, the use of a current mirror circuit would avoid this problem. Thus, in the current mirror circuit, one stage thereof could be coupled to the main channel amplifier such that a current proportional to the main channel signal flows therethrough, while second and third stages could be coupled to the switching circuit so that currents which also are proportional to the main channel signal flow therethrough. The currents of the second and third stages can be added to the recovered oppositely-phased sub-channel signals without presenting a load either to the main channel amplifier or to the switching circuit. Hence, the derived left and right channel audio signals are of greater fidelity.
The foregoing is but one application of a current mirror circuit; and various other applications and uses of the current mirror circuit are known. However, some disadvantages attend current mirror circuits which have been proposed heretofore. As one example of such a current mirror cricuit, each of the two stages is formed of a simple transistor whose base electrodes are connected in common. The collector electrode of one transistor is coupled to its base electrode, and both transistors have substantially the same current amplification factors and other characteristics. While the current through the emitter-collector circuit of one transistor is almost the same as the current through the emitter-collector circuit of the other transistor, the ratio between such currents is determined, in part, by the transistor amplification factor. As this current amplification factor increases, the ratio between the two currents approaches unity. However, since there is an upper limit to the value of the current amplification factor, there also is an upper limit to the approach of this current ratio to unity.
In one modification of this proposed current mirror circuit, the collector-base connection is effected through the emitter-base circuit of another transistor, the collector electrode of which is connected to a reference potential. The ratio between the currents flowing through the first and second transistors now is proportional to the product of the current amplification factor of either of these transistors and the current amplification factor of the third transistor. While the current ratio more closely approaches unity in this modified proposal, there still is a practical upper limit thereon which, in many instances and applications is not acceptable. That is, even in this improved proposal, changes in the current flowing through one transistor are not reflected entirely in the current flowing through the other transistor.